1. Field
The disclosed concept relates generally to optically variable devices (OVD's) and, more particularly, to OVD's with integral imaging systems comprising an array of focusing elements and a corresponding array of micro-objects that, when viewed through the lens array, change in appearance depending on the relative location from which the OVD is observed. The disclosed concept also relates to security devices that comprise such OVD's, articles that employ such security devices, and methods for creating such OVD's.
2. Description of Related Art
An optically variable device (OVD) is a visual device that creates a change or shift in appearance, such as, for example and without limitation, a change in color, when observed from different relative observation points. The evolution of the OVD as a security device stems largely from the search for a mechanism to resist counterfeiting of certain articles and products, or alternatively to render such copying obvious. For example, and without limitation, paper money, banknotes, certificates, security labels, product hang tags, drivers' licenses, ID cards, and credit cards, among other things, frequently employ one or more OVD's to resist counterfeiting or to verify authenticity.
A counterfeiting deterrent employed in some OVD's involves the use of one or more images that exhibit optical effects which cannot be reproduced using traditional printing and/or photocopying processes. In some instances, the images comprise holograms wherein when the OVD is viewed from a predetermined location, an optical effect results, such as, for example and without limitation, movement of the image. However, additional unique effects are continually needed to stay ahead of the counterfeiters' ability to access or simulate new imaging technologies. Accordingly, other security mechanisms having image-related optical effects have evolved over time.
One such optical effect is to exhibit at least one magnified version of an object or objects based upon the concept of moiré magnification, a phenomenon that occurs whenever an array of lenses is used to view an array of identical objects or elements of identical objects situated at the focal point of the lenses, the two arrays having approximately the same pitch. Moiré magnification is well known in the art and is related to the generation of integral images and to integral photography. As the lens array is aligned with the object array, a moiré pattern is observed in which each moiré fringe consists of a magnified image of the repeat element of the object array. As the arrays are rotated with respect to each other, the magnification and orientation of the image changes.
Typically, known OVD moiré magnification methods involve the steps of generating a plurality of micro-objects, selectively arranging the micro-objects, and providing an overlying layer of correspondingly arranged micro-focusing elements. The focusing elements are usually spherical or cylindrical lenses. Thus, such OVDs generally comprise a top lens layer, an intermediate substrate, and a bottom print or object layer which contains the micro-objects that are to be magnified or otherwise altered when viewed through the lenses. The micro-object layer typically comprises printed artwork. Conventional print technology limits the size of individual printed elements, which means that lens diameters of about 50-250 microns are the smallest that can practically be used in this configuration using conventional printing techniques. Using the lens types mentioned above at these diameters requires focal lengths of similar magnitudes (e.g., about 50-250 microns) in order to achieve adequate optical performance. Accordingly, OVD's having this configuration are too thick for many applications where a thinner security article is desired.
U.S. Pat. No. 5,712,731 discloses an OVD comprising an array of substantially spherical lenses having diameters in the range of 50-250 microns, and an associated array of printed micro-images. The lenses have diameters of 50-250 microns and typical focal lengths of 200 microns. The total thickness of the OVD, which depends primarily on the focal length, is about 250-450 microns.
Such a thickness is, however, not conducive for use with certain articles such as, for example and without limitation, banknotes, checks, security labels and certificates.
U.S. Pat. No. 7,468,842 discloses an integral imaging system having micro-objects formed by microstructured physical reliefs and a thickness of less than 50 microns. A physical relief, standing alone, is difficult to observe because there is no visual contrast between the high and low areas. This patent discloses techniques to create visual contrast in micro-objects formed from microstructured physical reliefs. For example, recesses in the reliefs can be coated with an opaque or colored material, or the reliefs can form optical structures that reflect or absorb light in particular regions.
There is still a need, however, for very thin OVD's having optical effects that are more sophisticated and provide a clear visual differentiation from existing optical security features and moiré magnification methods, and are hence more difficult to counterfeit, and for methods of making the same.